1. Field of the Invention
The present invention relates generally to the rapid switching of power transistors and, more particularly, to an FET-Bipolar switch which has the ability to turn a power transistor ON and OFF at a very high rate.
2. Description of the Prior Art
Bipolar transistors operating in the so-called "dump cycle" flyback mode at high frequencies (above 1 kHz) with inductive loads such as most brushless DC motor drives, require "proportional" base drives for operation at the highest efficiencies. The worst case operation of the above transistors is when the drive is PWM-controlled, where the transistors must be switched ON and OFF at much higher frequencies which are typically 1-20 kHz and must operate at close square-wave collector currents. In order to accomplish this, prior art base drive circuits require a base drive which starts with a current spike, followed by 1/.beta. times the collector current, and a high negative base current spike at turn-off, to promote fast turn-off. Such circuits are generally rather complex, expensive, and most of the transformer couple proportional base drive circuits of the prior art are useless in brushless DC motor drives, for example, as they cannot provide continuous base drive current at low RPM's. In addition, these circuits draw substantial base currents which results in a high average power supply current, which means increased cost and loss of an additional power supply.
In the prior art, it is known to use field-effect transistors (FET's) to drive bipolar switching power transistors. One such system utilizes one MOS hex inverter buffer, one low-voltage FET, and one high-voltage FET to switch a bipolar transistor. While such a circuit appears to function satisfactorily, it does not provide the desirable fast turn-off of the transistor inasmuch as it doesn't provide a good turn-on or turn-off current spike into the bipolar base. This limits the switching speed of the power transistor such that it cannot be used where very high speed switching is a requirement such as is the case in a brushless motor drive.